The superficial abdominal muscle group (hereafter "abdominal muscles") comprises four distinct but overlapping muscular structures: the external of descending oblique muscles located on the sides and forepart of the abdomen; the internal of ascending oblique muscles located beneath the external obliques; the transversalis muscles located beneath and the abdominal rectus muscles which the internal obliques; extend down the whole length of the front of the abdomen. Most of these contribute to trunk flexion. All contribute to abdominal constriction. Two contribute to trunk rotation to the right or left.
The most straightforward and generally useful movement to address the entire abdominal musculature is trunk or torso flexion. Such flexion includes flexion of the thoracic, lumbar and sacral regions of the spine, as well as flexion of the pelvis. Abdominal contraction also results in chest depression and some shoulder depression.
Various calisthenic or freehand abdominal exercises have long been practiced for the purpose of strengthening the abdominal muscles. Though leg raises, trunk twists and various other movements have been used, the primary exercise mode has been the tradition situp. The situp is, however, only partially effective in addressing the ventral torso below the ribs as the torso is flexed--chest bent toward the thigh.
Situps and leg raises involve trunk flexion, but such flexion is usually only supportive and secondary to hip flexion, i.e., bending the thighs onto the chest. Hip flexion involves the hip flexor musculature: the sartorius, iliopsoas, and rectus femoris. The abdominals are involved to stabilize the pull of the hip flexors against the bones in the torso, but the major emphasis of such exercises is on the hip flexor group, not the abdominals.
The trunk curl is a freehand exercise developed to work the abdominals with maximum exclusion of the hip flexors. The trunk curl is described in books by Ellington Darden, Ph.D., Nautilus Sports/Medical Industries, Dallas, Tx. Though the trunk curl is useful for teaching proper isolation of the abdominals, it lacks a counterforce applied to the back which is sufficient to achieve a meaningful stretch.
The same exercises have been performed with the additional benefits (and disadvantages) of dumbbells and barbells. Along with this evolution came the Roman Chair and the Slant Board, each of which possessed a method to lock down the feet or legs as the torso was worked from various angles of declination. While such approaches were very popular and at least partially successful for isolating and working the abdominal structures, they had problems. They were as much hip flexion exercises as they were abdominal exercises.
The early development of Nautilus.TM. equipment utilizing adjustable weight stacks, force redirectional sprocket and chain systems, and cams for applying variable resistance forces discouraged the development of equipment for exercising the abdominal muscles in an isolated manner. It was thought that because the abdominals participate to some extent in all exercises, especially pullover and chinning movements, they would be corecipients of the benefits of such equipment. A special machinery for the abdominals arose, however, in response to customer demand.
About 1980, the first Nautilus.TM. "abdominal machine" was introduced. It was a rotary-movement machine that offered the potential for meaningful isolation of the abdominals, but was mostly used as a situp machine, i.e., a machine to assist hip flexion movement. Such unintended use obviated the potential for isolating the abdominals and, in some cases, was thought to lead to back problems. This so-called "crunch" version of the Nautilus.TM. abdominal machine was not effectively usable by weaker individuals because the movement arm was heavy and not counterbalanced.
A less ideal, but more generally usable rotary-movement Nautilus.TM. machine was introduced about 1982. It was more of a situp machine that worked the hip flexors, so was readily accepted and mastered by end users because of the common situp mentality. Other companies copied this approach.
In 1986-87, several abdominal machine prototypes with a moving axis were developed, though not commercially produced. The spine is comprised of not one joint, but many, all of which rotate simultaneously. However, there exists only one effective and collective axis of the torso at any instant, hence, instant axis. This instant axis moves in an anterior-posterior reciprocation as the spine is alternatively flexed and extended. Therefore, the thought behind the moving axis machine was that a movement axis that remained concentric with the body's instant axis might force the user to flex the spine rather than the hips and perform a true abdominal exercise, not a situp movement.
In 1989, a new Nautilus.TM. machine was introduced that included elbow pads, a rotary movement similar to a limited range pullover, and a moving axis. This machine was, however, designed to severely limit the movement range toward stretch (extension) where much of the effectiveness of abdominal exercise is felt and often desired by the user.
While conventional design approaches to abdominal muscle exercise machines have provided some potential for isolated torso flexion/extension to achieve desired abdominal involvement, the correct function was not imposed on the user. It was entirely possible to work the machine's mechanism with incorrect actions, such as situp/hip flexions, because the proper torso flexion/extension was not mandated.